The reaction of an unsaturated substrate with carbon monoxide and hydrogen is known as hydroformylation. It has been disclosed in the past that glycolaldehyde, which is a useful intermediate for the preparation of ethylene glycol, may be prepared by a hydroformylation reaction of formaldehyde employing a rhodium catalyst. However, the preparation of glycolaldehyde in this way is hindered in that the rhodium catalyst also promotes hydrogenation of formaldehyde to methanol, lowering glycolaldehyde yields.
Attempts have been made to suppress methanol production and increase selectivity to the glycolaldehyde product and in this regard it has been found that good yields of glycolaldehyde can be achieved by use of rhodium catalysts containing aryl-substituted phosphine ligands such as triphenylphosphine, optionally in combination with protonoic acids (e.g. see Pure & Appl. Chem., Vol. 62, No. 4, pp. 661-669, 1990). However, such aryl-substituted ligands suffer from the disadvantage that they are unstable in the reaction conditions, lessening their effectiveness.
A further limitation on this method of preparing glycolaldehyde, in particular when it is to be used as an intermediate in the preparation of ethylene glycol, is that good results are only obtained when using para-formaldehyde in non-aqueous conditions and that use of the cheaper aqueous formaldehyde (formaline) gives lower conversion and selectivity to glycolaldehyde. This is thought to be due to the instability of the catalyst in aqueous conditions. Indeed, the difficulty in hydroformylating aqueous formaldehyde represents a major obstacle to the commercialisation of this approach for the production of ethylene glycol.
European patent application EP-A-0331512 reviews the use of a rhodium-phosphine ligand complex, wherein the phoshine ligand is a triorganophoshine, in the hydroformylation of aqueous formaldehyde to glycolaldehyde, which can then be used to prepare ethylene glycol.
A process has now been developed for hydroformylating formaldehyde that is based on the use of a rhodium catalyst and a specific form of phosphine ligand. The process has performance advantages when compared to known rhodium catalysts having aryl-substituted phosphine ligands. Moreover, the catalysts of the present invention are more stable in aqueous conditions than catalysts containing aryl-substituted phosphine ligands, and may readily be used to convert aqueous formaldehyde to glycolaldehyde.